1. Field of the Invention
This invention relates to a method of clamping wires during wire bonding operations in the fabrication of semiconductor circuits.
2. Brief Description of the Prior Art
In the fabrication of semiconductor devices, wiring between two bond pads on a chip or between a bond pad on the chip and an external bonding location, such as a lead frame, is generally performed by making a ball bond on one bonding pad at one end of a wire fed from a spool and a stitch bond at the other end of the wire. The wire is also severed from the spool from which it has been fed at the stitch bond location. Such bonding and wire severing is generally performed with the aid of a capillary through which the wire to be bonded is passed from the wire spool. The procedure is to form a ball from a portion of the wire which extends out of the capillary, bond the ball to a bond pad and move the capillary to the second bond pad with the wire being fed out through the capillary during travel of the capillary to the second bond pad. The wire is then stitch bonded to the second pad, usually a lead finger, and prepared for severing from the spool, using the tip of the capillary to perform these tasks.
In order for the capillary to perform the function of stitch bonding, the capillary must apply a force against the wire which rests on the bond pad/lead frame. In order to perform this task without fracture of the capillary, as a first condition, the capillary wall must have sufficient thickness to withstand the forces applied thereto at the location of force application during the stitch bonding procedure. This has been accomplished in the prior art by providing a wire bonding capillary with central bore or aperture, a circular cross section and wall thickness from exterior surface to central bore sufficient to accomplish the above described purpose and withstand the forces thereon.
With the continual decrease in the spacing dimensions between wire bonding locations, particularly in the semiconductor art, the problem of bonding wire to a wire bonding surface with a capillary and then moving the capillary to a new location without interference with adjacent wire bonding locations and wires bonded at adjacent wire bonding locations has become an increasing problem. As the dimensions decrease, the possibility that the capillary will interfere with or strike an adjacent bond pad or wire extending from an adjacent bond pad in its travel from one bonding location to a second bonding location increases.
A prior art technique that has been developed to accommodate and/or minimize this problem has utilized a capillary with the diameter of the tip portion decreased. This results in a reduction of capillary wall thickness and provides a poor stitch bond. Accordingly, this technique is undesirable.
A further prior art technique that has been developed to accommodate and/or minimize this problem has utilized a wire bonding capillary of circular cross section with a portion of the capillary wall on a pair of opposing sides of the capillary removed to provide an indentation thereat. Capillaries of this type are provided by Texas Instruments Incorporated under the trademark BowTI.TM.. This is accomplished by using a capillary having a nose or stitch face somewhat in the shape of a figure "8" with a hollow center to carry the wire as in the prior art and with an enlarged but thin walled waist region. A capillary with this shape is still capable of performing the functions of forming and bonding the ball from the wire passing therethrough at one pad and then stitch bonding the wire at a second pad, using the thicker-walled portion of the "8"-shaped capillary The top and bottom portions of the "8" must be used to make the stitch bond because they are thicker and better capable of withstanding the forces applied to the capillary during stitch bonding. With a capillary having the above described shape, bonds are to be made at very closely adjacent bond locations or at designated bond locations where a particular one of two available capillary orientations must be used. These capillary orientations are at an angle of 90.degree. relative to each other so that the circular portion of one capillary fits into but is separated from the waist portion of the adjacent capillary or so that the capillary does not strike some impediment in the travel path. After ball bonding one end of the wire extending from the capillary, the capillary with wire therein is moved to the next bonding location during which time the wire is passed through the capillary. The other end of the wire just bonded is then stitch bonded at the next bonding location, generally a lead finger of a lead frame, using the thicker portions of the capillary. The particular order of the bonds and the determination which bonds will be made by which of the two capillaries is predetermined and stored in a controller which controls the operation of the capillaries and forms no part of this invention. Such controllers are known in the art.
Prior to formation of stitch bonds to the lead frame finger, a clamp is brought down over the lead frame fingers and spaced from the end portions of the lead frame fingers to which the bond will be made to position the lead frame fingers against a support thereunder. Stitch bonds (as well as ball bonds to bond pads on the chips) are then made to the end portions of selected lead frame fingers with the capillary which moves in one of the x-direction or y-direction. The clamp is then moved out of contact with the lead frame fingers, concurrently causing some flexing of the fingers, this being detrimental to the wire loops that have been formed by causing some undesirable movement of the wires away from their desired location. The die is then moved to a second station the lead frame fingers are again clamped and the remaining stitch bonds are then made with the capillary oriented in the other of the two orientations. The clamp is then removed from the lead frame fingers a second time, causing flexing of all of the fingers for a second time. The problems with such flexing are that the wires bonded to the flexed lead fingers tend to sag and/or move sidewise. Due to the very close tolerances available in view of the small dimensions, the possibility of a short circuit or contact with another wire or the like is materially increased.